DESCRIPTION: Cell based regenerative therapy requires the activity of stem progenitor cells for tissue regeneration. Connective tissue progenitors (CTPs) are defined as the heterogeneous population of tissue resident stem and progenitor cells that are capable of proliferating and differentiating into one or more connective tissue phenotypes. The number and heterogeneity of CTPs can be estimated using the colony formation assay, in which colony founding CTPs are detected and measured by their ability to proliferate and colony of progeny that express a connective tissue phenotype in vitro. In addition to measuring the number and prevalence of CTPs in a tissue sample, differences in biological potential between various CTPs can be assessed based on variation in cell number, distribution, morphology and expression patterns between colonies that are formed. While very useful, a very important limitation of traditional colony formation assays is that their only direct measurement is made on the progeny of the founding CTP (the colony). Information about the founding CTP can only be inferred from the number of colonies and their features. Colony assays to not reveal the physical or biological properties of the rare individual colony founding CTPs themselves, before proliferation. To address this limitation, we have developed a live cell, videomicroscopy system that enables systematic characterization of colony founding cells (morphology, expression of surface markers) before colony formation. Using multicolor labeling methods, a mixed tissue derived cell population is labeled for cell markers of interest at the time of plating. Time lapse phase contras images are then captured over 6 days, while colonies form, over a large field of view. At the end of the culture period, colonies are characterized with respect to established markers and colony metrics. By reversing the video, each colony can also be traced back to identify the colony founding cell, as one in a sea of other non-progenitors. Using this method we propose to significantly advance our understanding of the nature of human CTPs, their diversity and subtypes, their location in bone tissue and define factors that can be measured non-invasively and used to predict their biological potential, or select CTP best suited for specific therapeutic purposes.